Actually, the odd operator is =, not +=. Most operators will dictate the context of their operands, and so does +=: both operands are in scalar context. But = is special cased, there the operands could be either in list or scalar context - and it's the left operand that decides.

The following program might be of interest:

#!/usr/bin/perl

use strict;
use warnings;

my ($a, @a) = ("");

sub foo : lvalue {
    print wantarray ? "LIST:   " : "SCALAR: ";
          wantarray ?  @a        :  $a
}

 foo ()   = 1; print "\$a = $a; \@a = [@a]\n";
(foo ())  = 2; print "\$a = $a; \@a = [@a]\n";
 foo ()  += 3; print "\$a = $a; \@a = [@a]\n";
(foo ()) += 4; print "\$a = $a; \@a = [@a]\n";

__END__
SCALAR: $a = 1; @a = []
LIST:   $a = 1; @a = [2]
SCALAR: $a = 4; @a = [2]
SCALAR: $a = 8; @a = [2]
[download]

Abigail